Create short permalinks similar to Stack Overflow's "short permalink to this question"

Question

I think I might already understand how this works, but wanted to be sure.

I am in the process of defining the routes for a new ASP.NET MVC application. I'd like to create short permalinks similar to Stack Overflow's short permalink to this question:

http://stackoverflow.com/q/4047890/61654

What route and controller mechanism is Stack Overflow using for this permalink behavior?

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